FIGURE 12-7 Endosymbiont hypothesis for the evolutionary origin of mitochondria and chloroplasts. Endocytosis of a bacterium by an ancestral eukaryotic cell (step 1) would generate an organelle with two membranes, the outer membrane derived from the eukaryotic plasma membrane and the inner one from the bacterial membrane (step 2). Proteins localized to the ancestral bacterial membrane would retain their orientation, such that the portion of the protein once facing the extracellular space would now face the intermembrane space. For example, the F₁ subunit of ATP synthase, localized to the cytosolic face of the bacterial membrane, would face the matrix of the evolving mitochondrion (left) or chloroplast (right). Budding of vesicles from the inner chloroplast membrane, such as occurs during development of chloroplasts in contemporary plants, would generate the thylakoid membranes with the F₁ subunit remaining on the cytosolic face, facing the chloroplast stroma (step 3). The organelle DNAs are indicated. Membrane surfaces facing a shaded area are cytosolic faces; surfaces facing an unshaded area are exoplasmic faces.